Visible cork insulation on a cathedral ceiling.
I would like to advise my sister on how to insulate there cottage cathedral ceiling , climate zone 6, on Christian Island, Great Georgian Bay, Canada, The cottage is only used from April until October.
The problem is overheating on the south facing roof, one pitch is 3/12 and the other is about 6/12. Currently, from the interior, you look up at the rafter bays and see the plywood roof deck and the roofing nails coming through, used to attach the composition shingles. You can feel on your face, the heat radiating from the underside of the roof. The loft area on a sunny day becomes to hot to use.
I was thinking they could press fit and glue with construction adhesive 2″ or 3″ thick cork to the under side of the plywood, on the south face only, and call it done. I think that would work for the aesthetic of the cottage.
I am not that confident regarding the craftsmanship of the comp roof, (however, there are no visible signs of leaks), so I think allowing the plywood to dry is important. My thought was that the cork would still allow that to happen.
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I'll leave others to comment on the usefulness of your proposed solution, but the first thing i'd check is the flame-spread rating of the cork insulation. Any finished materials you use on ceilings are limited by the building code to those of a certain rating for safety reasons.
I assume that the idea here is that you are looking for a quick fix. Unfortunately I have no experience with or knowledge of cork insulation, except that it has an R-value similar to fiberglass, I believe. My gut tells me that if the space is uncomfortable to the point that it is unusable, a few inches of cork are not going to make an appreciable difference. I'll be interested to see what others have to offer for a quick fix. In the meantime, here's an article about how to insulate cathedral ceilings: http://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
3" of cork is about R10, that would definitely take the edge off solar radiation. Not the cheapest solution though, you are probably looking at close to $10/sqft, the cork tiles I've used come typically in 12"x36", getting them to fit between 16OC rafters will not be easy.
For the money and time it would take, you are better off with 2" of polyiso under the rafters and covering it with t&g or plywood. Felt faced polyiso is vapour permeable and would still allow drying toward the inside.
For a summer cottage even some R14 batts with a decent warm side air barrier (drywall or plywood with taped seams) would work. If you leave a vent space above the batts, you can always add in eave and ridge vents in case you want to winterize the cottage down the road.
Thermacork, available here http://www.smallplanetsupply.com/thermacork, comes in 19.5" x 39" pieces, of varying thickness, at about R-4/in, so 3" would be R-12. That will definitely make the interior space more comfortable. It is vapor open and naturally anti-microbial, so a good match for your situation. You won't be able to get a perfectly tight fit against the rafters, though, so you might want to plan on cutting the pieces a bit small, filling the spaces with canned foam, and covering the gaps with trim.